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Bureau of Mines Information Circular/1979 

;^7 




In-Mine Evaluation of Underground 
Fire and Smoke Detectors 

By Russell E. Griffin 



UNITED STATES DEPARTMENT OF THE INTERIOR 



V 



Information Circular 8808 

In-Mine Evaluation of Underground 
Fire and Smoke Detectors 

By Russell E. Griffin 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Cecil D. Andrus, Secretary 

BUREAU OF MINES 

Lindsay D. Norman, Acting Director 



Ta/^'. 



is- 



.11 



'U ^^, /1'?f 



This publication has been cataloged as follows: 



Griffin, Russell E 

In-mine evaluation of underground fire and smoke detectors. 

(Bureau of Mines information circular ; 8808) 

Bibliography: p. 24-25- 

L Mine safety— Equipment and supplies. 2. Fire detectors. I. 
Title. II. Series: United States. Bureau of Mines. Information circu- 
lar ; 8808. 



TN295.U4 622'.08s [622'. 8] 79-16184 



CONTENTS 

Page 

Abstract 1 

Introduction 1 

Fire signatures 2 

Aerosols 2 

Energy 2 

Gases 2 

Optical view field flame detectors 4 

Ultraviolet sensing detectors 4 

Infrared sensing detectors 4 

Heat sensors (direct contact detectors) 6 

Products of combustion detectors 8 

lonization-type detectors 8 

Photoelectric -type detectors 9 

Solid-state detectors 10 

Electrochemical detectors 11 

Optical gas detectors 11 

Environmental considerations 12 

Bureau of Mines experience 17 

Shaft installations 17 

Underground fueling area installation 21 

Additional underground tests 22 

References 24 

ILLUSTRATIONS 

1 . Sizes of airborne contaminants 3 

2. Conceptual fuel storage area prototype fire detection-suppression 

system 15 

3. Conceptual underground maintenance shop prototype fire detection- 

suppression system 16 

4 . Smoke , gas , and heat sensors 19 

5 . Underground metal mine fire sensor package diagram 23 

TABLES 

1 . Characteristics of different photodetector devices 5 

2. Average contaminate ratios for various underground activities 13 



IN-MINE EVALUATION OF UNDERGROUND FIRE AND SMOKE DETECTORS 

by 

Russell E. Griffin 1 



ABSTRACT 

The current state-of-the-art of fire and smoke detection technology is 
reviewed from the standpoint of suitability for use in underground metal and 
nonmetal mines. Detection modes, fire signatures, and environmental considera- 
tions are included. Preliminary results of long-term in-mine tests are 
presented. 

INTRODUCTION 

Underground mines are becoming deeper, more spread out, and more mecha- 
nized leading to increased combustibles loading, and restricted miner egress. 
Welding, torch cutting, and electrical shorts account for over 50 pet of fires 
in noncoal mines since 1945. Spontaneous combustion is another common cause 
of fires in mines and, as ventilation systems become more complex, this hazard 
will probably increase. The Bureau of Mines activities and programs in health 
and safety have addressed this problem and found one of the key needs is reli- 
able sensors for early smoke and fire warning. This report describes sensor 
characteristics needed to provide rapid and reliable response to fire and 
smoke, the need for minimizing false alarms along with providing maximum reli- 
ability and low maintenance. Bureau experiences with sensor applicability and 
use in underground mine environments are also described. 

Classification of the various smoke and fire sensors into three 
categories--optical view field, direct contact, and products of combustion 
(POC) --requires some explanation since most off-the-shelf devices (systems) 
have more than one principle of response and may respond to more than one param- 
eter. Because hybrid sensors may include heat detectors as well as photo- 
electric and/or ionization- type POC detectors, classification is according to 
the primary intended use. For the nonconventional fire detectors that have 
been developed for gas analysis and detection, the classification may be less 
obvious. Most of the nonconventional detectors are described in this report 
under POC types of sensors, even though they may depend on optical- or 
filament- type principles. A further distinction of detector types divides 
them into point (or local) source, as opposed to the extended area-type sensor. 

■■■Electronics engineer. Twin Cities Research Center, Bureau of Mines, Twin 
Cities, Minn. 



For more detailed information on the types of fire detectors available, 
there are good survey articles (£, 14) , National Fire Protection Association 
(NFPA) reference books (10) , and manufacturers' sales literature. 

FIRE SIGNATURES 

Any product of a fire that changes the ambient conditions is called a 
fire signature (£) , and is potentially useful for detection. To be practical 
for detection, a fire signature must cause a measureable change in some 
ambient condition. With other factors being equal, such as hardware costs and 
detection times versus hazard level, the preferred fire signature is one that 
generates the highest signal-to-noise ratio in the earliest period of the fire 
development. The principal fire signatures used in the detectors discussed 
herein are aerosol, energy release, and gas signatures » 

Aerosols 

Aerosols are particles suspended in air. The process of combustion 
releases large quantities of solid and liquid particles into the atmosphere 
ranging in size from 5 X 10 to 50 um (fig. 1) (4). Aerosols resulting from 
a fire represent two different fire signatures. Particles less than 0.3 um 
do not scatter light well and are classified as invisible. Those larger than 
0.3 um do scatter light and are classified as visible. The invisible aerosol 
signature is usually referred to as "products of combustion" and the visible 
aerosol signature as smoke. Invisible aerosol is the earliest appearing fire 
signature in most cases. Coal can evolve CO before particulates--polyvinyl 
cholride (PVC) materials evolve HCL quite easily. 

Energy 

Fire also constantly releases energy into the environment providing some 
useful fire signatures. The earliest energy signatures detectable with avail- 
able hardware are the infrared (IR) and ultraviolet (UV) signatures. Except 
for highly unsaturated hydrocarbons such as acetylene, infrared emissions from 
hydrocarbons are particularly strong in the 4.6-iJ;m region due to carbon diox- 
ide and in the 2.7-um region due to water vapor and COg. This radiation signa- 
ture can be used effectively for detection but there is the possibility of 
noise from manufactured IR sources. Ultraviolet fire signatures appear in 
flames as emissions from OH, COg , and CO in the 0.27- to 0.29-(i.m region. 

Gases 

Many gases are added to the atmosphere during a fire that are called 
evolved gas signatures. A related change is the reduction of oxygen content 
(the oxygen depletion signature). Evolved gases may include CO, COg , HCl, HCN, 
HF, HgS, NH3 , and NOx, depending on the type of material burning. The most 
useful gas for detection is CO, since it is present in almost all fire situa- 
tions. Slow-burning and fuel-rich fires in particular produce large quantities 
of CO. 

Underlined numbers in parentheses refer to items in the list of references at 
the end of this report. 



Aerosols 



Normal impurities-in quiet outdoor air 



Metallurgical dust and fumes 



Fog 



Smelter dust and fumes 



Ammonium chloride fumes 



Mist 



Foundry dust 



I Flour mill dust 

Alkali fumes sprayed zinc dust 



Zinc oxide fumes 



Tobacco Tobacco Virus 
mosaic necrosis and 
virus virus protein 

1 



T 



Carbon black 



Tobacco smoke 



H2O-NH3 

H« ^2^ Diameter of gas molecules 
' ooooo« = ^ 

NsCOj 



Oil smoke 



Magnesium oxide smoke 



Resin smoke 



Silver iodide 



(Enamels) 



Combustion nuclei 



■♦ Range of sizes 

—^ Small range average 

— Doubtful values 



Sea salt nuclei 




1 V ' 



Raindrops 



Ground limestone 



Sulfide ore, pulps for flotation 



Sulfuric acid mist 



Condensed 
zinc dust 



Cement dust 



Pulverized cool 



Insecticide dusts Plant spores 



Bacteria 



Pigments (Flots) 



Pollens 



Sneezes 



Fly ash 



Sand tailings 



Spray dried milk 



Reference 
sizes 



Washed foundry sand 



Human hair diometer 



Visible to eye 



400 325 200 100 65 48 35 10 

Screen 1 >-, — l^ — 1 — 1 — i-r-i — 1 

mesh 



aoooi 0.0005 0.001 0.0050.01 0.05 0,1 0.5 1 5 10 

PARTICLE SIZE, /im 

FIGURE 1. - Sizes of airborne contaminonts 



50 100 500 1,000 5,000 10,000 



OPTICAL VIEW FIELD FLAME DETECTORS 

Fire detection devices of this type respond to radiant energy portions of 
the electromagnetic spectrun generated during flaming combustion of materials. 
The principal sensing elements used include solid-state detectors--junction 
and bulk effects types, vacuum or gas filled tubes, and thermocouples and 
thermistors for special applications. The photodetector family type fills a 
large part of the category; table 1 gives a summary of their operating 
principles . 

Ultraviolet Sensing Detectors 

The variety of UV detectors (wavelengths less than 0.4 um approximately) 
is small compared with those in the IR region because of the basic problems 
associated with UV detection. Ordinary glass windows cutoff radiation below 
0.3 Um and quartz and UV-grade sapphire become opaque below 0.18 um. Below 
0.1 um there is essentially no suitable window material. 

Detector Electronics Corp. (Det Tronics Corp.)^ manufactures a basic UV 
detector designed to operate in the 0,18- to 0.2450-um region. It is insensi- 
tive to both sunlight and artificial light, and has a 90° cone of vision. The 
quoted sensitivity is 0.093 m flame from 4.57 m. The sensing element is a 
gas filled, UV-sensitive tube operating on the Geiger-Mueller principle. 

Fenwal Corp. produces a UV detector responding to radiation in the range 
of 0.19 to 0.25 um that also operates on the Geiger-Mueller ionization princi- 
ple similar to the Det Tronics model and has a 120° cone of vision. Response 
time is given as 15 msec for a propane-air flame 44.5 mm high at a distance of 
0.200 m from the flame source. Other suppliers include McGraw-Edison Co., and 
Pyrotector, Inc., with similar spectral responses and operating on the Geiger- 
Mueller principle. The Pyrotector device incorporates a signal delay circuit 
(3 sec) to minimize false alarms from sparks and lightning. 

Infrared Sensing Detectors 

Infrared detectors have the problem of background radiation at ambient 
temperatures 86° F (25° C) being entirely in the IR region (wavelengths 
greater than 0.8 um) , A common method of discriminating against this back- 
ground is that of chopping the incident radiant flux so that the detector 
receives only a fixed radiant frequency, typically 4 to 30 Hz. Combinations 
of optical filters and mechanical scanners have also been tried to narrow the 
signal wave length along with the frequency discrimination. 

Infrared detectors are classified according to their method of response 
to heat and photon flux: thermal detectors and quantum detectors. 

Thermal detectors respond to energy absorbed by a temperature-sensitive 
material or an absorbing film in contact with the temperature-sensitive 

3 Reference to specific equipment, trade names, or manufacturers is made for 
identification only and does not imply endorsement by the Bureau of Mines, 



TABLE 1. - Characteristics of different photodetector devices 

Photoemissive Vacuum tubes in which light impinges on a metal 

cathode, releasing 1 electron per photon of 
light. Photomultiplier plates are often con- 
tained in the same tube envelope. Standard 
for sensitivity comparison. Has poor long- 
term stability and high quiescent power 
consumption, needs high voltage power supplies, 
and has poor shock and vibration resistance 
characteristics . 



Photovoltaic, 



Absorbs light and produces an output voltage; 
does not need an external power supply. Most 
common materials are silicon and selenium. 
Selenium exhibits good response in the UV and 
is inexpensive; however, it exhibits hysteresis 
to light. Silicon photodetectors show promise 
in replacing selenium as cost declines. Does 
not exhibit serious hysteresis and has micro- 
second response times. 

The silicon solar cell optimized for resist- 
ance to nuclear radiation (n junction on top), 
exhibits poor response in the UV, a broad band 
response in the visible, and a peak in the 
near IR. Silicon photocells with enhanced 
blue response are now available. 



Photoconductive junction type 
(sometimes called photosensitive). Conductivity changes as the device absorbs 

light. Reverse biasing of photovoltaic 
junction photocells leads to operation in the 
photoconductive mode. Silicon is again the 
most popular material for junction photo- 
conductive cells. For reverse biasing PllSr 
processing is preferred to the conventional 
PIT junction. PIN photodiodes operated with 
reverse bias can have response times as fast 
as 1 ns . Light history effects are absent 
in PIN cells in either of the photovoltaic 
modes . 

Bulk-effect photoconductive cells.. Behave like resistors whose resistance 

decreases nonlinearly with an increase in 
light intensity. The usual materials are CdS 
and CdSe. Usually have sharp-peaked spectral 
responses unless they are specially compen- 
sated. Require a low voltage power supply. 
Major disadvantages are slow response and 

light hysteresis. 

1 PIN junction is the region of transition between p-type and n-type with the 
diffusing process so controlled that a thin "intrinsic" region separates the 
n and p regions . 
PN junction is the region of transition between p-type and n-type material in a 
single semiconductor crystal. 



material. Included in this category are such devices as thermocouples, metals 
or semiconducting layers with resistance a function of temperature (bolometers 
and thermistors), pyroelectric detectors whose polarization is temperature 
dependent, gases with pressure being temperature dependent, and thermopiles 
with an output of electromotive force. 

Quantum detectors respond to photon flux falling on a sensing element 
and exciting electrons in a bound state to a free or conducting state. 

Within these broad classifications the IR spectrum is divided into three 
divisions: near infrared (0.8 to 1.4 um) , intermediate infrared (1.4 to 7 um) , 
and far infrared (>7 |i,m) . Near infrared requires conventional silicon and ger- 
manium detectors; intermediate uses special IR detectors; and far infrared 
calls for thermal detectors such as thermocouples, thermistors, and thermo- 
piles. In some cases there is overlapping spectral area of response. Lead 
sulfide is one of the most versatile photoconductors and responds to IR from 
approximately 1 to 4 um. Doping Ge with Hg and Cu extends its response to 
8 to 14 [im. Sensitive areas range from 0.01 to 10 mm^ , with time constants 
shorter than 1 ns . (InSb may be more useful than PbS since it covers the CO 
and COg bands but must be operated in a cooled mode, 77° K.) 

Pyroelectric -type detectors are a more recent development (1^,3). These 
consist of a slice of ferroelectric material sandwiched between electrodes to 
provide a sensing cell. Typical materials include triglycine sulfate, tourma- 
line, Rochelle salt, barium titanate, and polyvinyl flouride. Electrically, 
the material behaves similarly to a capacitor dielectric with a strong 
temperature -dependent polarization of the magnetic domains. One of the elec- 
trodes is transparent to IR radiation which causes a small temperature 
increase producing a polarization charge on the pyroelectric material and a 
corresponding voltage change across the two electrodes of the cell. Victory 
Engineering Corp. (VECO) manufactures commercially available pyroelectric IR 
detectors of triglycine fluoroberyllate and triglycine sulfate requiring no 
cooling, and operating at ambient temperature ranges from -10° to 60° C, and 
-10° to 40° C, respectively. Standard spectral response is from 1 to 45 \ira. 

Pyrotector, Inc., produces a near IR flame detector that responds in the 
spectral range of 0.65 to 0.85 um. The detection cell is a dual element, solid- 
state photoresistive device with appropriate optical filters. The discrimina- 
tion portion responds to wavelengths of 0.4 and 0.55 um. The two elements 
function as a spectral voltage divider to detect flame and discriminate 
against ambient light from sunlight and incandescent or fluorescent lighting. 
The manufacturer quotes its sensitivity range as 6.1 m with 0.093 m^ of hydro- 
carbon fire at 10 ft-c ambient light level (higher ambient levels decrease its 
sensitivity), with a 120° cone of vision It is recommended for use in low 
ambient light levels such as would be found in an underground mine. 

HEAT SENSORS (DIRECT CONTACT DETECTORS) 

There are two general types of heat sensors; those that employ the fixed 
temperature principle, and those that employ the rate-of-rise principle (10). 
With the fixed temperature approach an appropriate temperature level setting 



is selected. When the active element is heated to its operating temperature 
the active element will bend, expand rapidly or maximally, fuse (change its 
electrical conductivity and physical state), or produce an electromotive force 
(emf) that can be amplified to actuate an alarm. Commercially available 
devices include temperature -sensitive ampoules or pellets, bimetallic elements, 
eutectic solders and salts, snap disks, thermocouples, and thermistors as 
point sensors. There is a subcategory of continuous line or wire types using 
thermistor material, eutectic salt, pressurized gas, or twisted insulated 
wires under tension. Excluding specially prepared rapid response thermo- 
couples, a major disadvantage of these fixed temperature types of sensors is 
slow response time because of thermal inertia. Another disadvantage is their 
inability to detect low level incipient combustion in its early stages. Com- 
mercial fire detector thermocouples have response times on the order of a few 
tenths of a second. Advantages of direct contact detectors are low cost, 
reliability, and insensitivity to vibration and dust -laden atmosphere. Con- 
tinuous line heat sensors have been tried in underground mines . Companies 
that supply this type of heat sensor are Fenwal, Walter Kidde, Protectowire, 
Systron-Donner , and McGraw -Edison. Their systems were developed for use in 
aircraft engine compartments and military vehicles and have been adapted for 
use in mines and mine vehicles. 

The sensing element in the Fenwal system is an Inconel tube packed with a 
thermally sensitive eutectic salt and a nickel wire center conductor. The 
application of heat at any point along the element length causes the resist- 
ance of the salt to drop sharply at the eutectic temperature, causing an 
increased flow of current through the element. This increase in current flow 
is sensed by a control unit, producing an output signal to actuate an alarm. 
One possible problem with a eutectic salt is polarization of the material if 
it is used with dc voltage. 

The Kidde and McGraw -Edison sensing elements contain a thermistor (semi- 
conductor) type material whose electrical resistance drops with temperature 
increase. The behavior of the semiconductor material may be altered during 
manufacture to provide various ranges of temperature characteristics to meet 
different application needs. In use the cable actually performs as a 
temperature -averaging device, its absolute resistance being a function of its 
surrounding ambient temperatures. When the resistance value reaches the set 
point of the control system an alarm is signaled. Both the eutectic salt and 
the semiconductor types reset themselves upon removal of the fire condition 
and are rugged enough to withstand severe vibration and shock. These systems 
can also be connected for supervisory monitoring. 

The Systron-Donner system sensing element is a small -diameter swaged tube 
containing a special center wire that contains an adsorbed gas. Upon heating, 
the gas is released and its pressure actuates a diaphragm switch attached to 
one end of the tube. The switch, in turn, energizes an alarm system. This 
system also resets itself and has supervisory monitoring capability. 

The Protectowire line detector is comprised of two actuator wires individ- 
ually encased in a heat -sensitive thermoplastic material. The encased wires 
are twisted together to impose a spring pressure between them. This assembly 



is then spirally wrapped with a protective tape and finished with an outer 
covering to suit the environment of use. The line detector is connected in 
series to a suitable power supply, a supervisory relay circuit, and an end- 
of-line resistor, establishing a low-level monitoring current through the 
system. A break in the actuator, or loss of power, opens the supervisory 
relay, in turn producing a trouble signal from a second power source. At the 
operating temperature of the sensor the plastic yields to the inherent pres- 
sure allowing the actuator wires to contact each other, increasing the current 
flow in the system, and actuating an alarm or other function. This type of 
sensor does not reset itself and any portion subjected to enough heat to 
actuate the sensor must be removed and a new section spliced in. The lowest 
operating temperature available is 155° F (68.5° C) , somewhat lower than the 
previously discussed wire sensor types, and making it ideal for mine use. The 
wire is resistant to moisture, chemical fumes, and other deteriorants and can 
be obtained in a waterproof version. 

Rate -of -rise detectors are designed to respond to changes in temperature 
at a rate of approximately 15° F/min (8.3° C/min) . They are fairly reliable 
and do not alarm for slow increases in ambient temperature. They are not 
suitable for smoldering fires and locations where rapid changes in temperature 
occur as part of ambient conditions. 

A common way to obtain a rate -of -rise system is to use a sensor with a 
voltage output connected to an electronic differentiating circuit. Fenwal and 
the Notifier Corp. combine the fixed temperature principle with the rate -of 
of-rise. Two bimetallic elements with different coefficients of expansion are 
used. For very low rates of change both materials line up evenly and operate 
as a fixed temperature level device. For rapid rates of rise the materials do 
not expand evenly, producing an alarm even though the fixed alarm temperature 
is not reached. 

PRODUCTS OF COMBUSTION DETECTORS 

Products of combustion in fires include solid particulates and liquid 
mists (including invisible and visible particle sizes), ionized species gases, 
and radiant energy. Combustion product detectors sense one or more of these 
constituents excluding heat or flame. 

Ionization -Type Detectors 

lonization-type detectors constitute one general class of POC detectors. 
The most common source of ionization is an alpha emitter such as Ra^^segQ^ ^ 
or Am^*^. Beta emitters are sometimes used but not as often since stronger 
sources are needed to create ionization currents similar to those produced in 
alpha detectors. Kr^^ is one beta emitter that is used. The radioactive 
emitter ionizes the air in a chamber between two electrodes. Current in the 
picoampere range is produced from production and transport of positive and 
negative ions to the opposite poles of the plates. A decrease in current, 
relative to clean air, is obtained when combustion products enter the chamber 
because the ionized combustion particles are larger and heavier than the air 
molecules and move more slowly toward the end of the chamber. An electronic 
circuit detects the drop in current and initiates the alarm. 



Ionization -type detectors are also responsive to particulate matter from 
sources other than fires. If the sensitivity is set too high they will respond 
to cigarette smoke, room deodorant sprays, dust particles, and airborne partic- 
ulate matter resulting from cleaning chemicals reacting with ammonia. Tests 
have shown that they will not react readily to certain slow-burning plastic 
fires producing large quantities of smoke (13) . The ionization-type detector 
will not work properly in regions with high radiation backgrounds, and they 
depend on an airflow velocity adequate to cause the particulates to enter the 
ionization chamber but not so large as to push them through too rapidly. Cham- 
ber design is a factor here and the chamber electrodes may need periodic clean- 
ing to remove settled dust. Most ionization-type detectors utilize the dual 
chamber design concept wherein one chamber detects the presence of combustion 
products, the other serves as a reference for sensitivity stabilization to 
environmental changes in temperature, humidity, and pressure. 

There are many sources of ionization detectors but few of these are of an 
industrial quality and ruggedness to warrant testing in a mine. One of the 
more promising ones from the environmentally rugged standpoint is the Becon 
Mk II, developed and produced by the Anglo American Electronics Corp. of South 
Africa. It is a single ionization chamber -type, beta ray source unit devel- 
oped specifically to withstand the environmental and operational conditions 
found in deep level gold mines. Such conditions include: corrosion, tempera- 
tures up to 104° F (40° C) , air velocities up to 6 m/sec, relative humidities 
of 100 pet, and dust. The relatively low-cost device is also claimed to have 
long life, ease of installation, with little or no maintenance, and low power 
requirements . 

The Bureau of Mines has also produced a prototype ionization fire 
detector suitable for mine use (8) . A primary charging current of unipolar 
air ions is generated between two cylindrically concentric electrodes, and 
forced airflow through the instrument carries submicrometer particulates 
through the unipolar region where they acquire charge. The charged particles 
impinge upon a third electrode downstream, producing a current directly pro- 
portional to the particulate concentration. Two unique features of this 
device are a filter and cyclone assembly to protect the ionization chamber 
against, ambient dust, and additional electrodes that provide a means of deter- 
mining particulate size distribution in terms of their mobility. This unit is 
currently being evaluated in underground mine testing. 

There are other devices or techniques that utilize the ionization effect 
to sense the presence of particulates or gases, but these are used in labora- 
tory procedures and are not developed for ready use as POC detectors in a 
mining environment. 

Photoelectric-Type Detectors 

Photoelectric fire and smoke detectors constitute another general type of 
POC detector. The requirements are a source of light and a detector of that 
light to measure its radiant power. Four different modes of operation are 
possible depending on the amount of light transmitted or absorbed by the 
medium, the amount reflected, scattered, or refracted. Most, if hot all. 



10 



commercially available units possibly suitable for mine use are of the light - 
scattering type. A beam of light from a source travels across a light-tight 
chamber to a light trap or collector opposite the source. A photocell posi- 
tioned at right angles to the beam senses no light as long as the air inside 
the chamber is clean. The chamber is open to its surrounding atmosphere 
through baffling, and if smoke (POC) enters the chamber, light from the beam 
is reflected or scattered in all directions. Some of this scattered light 
reaches the photocell, changing its resistance and, by suitable electronics, 
initiates an alarm. The alarm level is adjusted to actuate at a given smoke 
concentration level (normally 3.3 to 6.6 pet m"-"- or 1 to 2 pet ft"''"). In a 
few models the electronics is also designed to give an alarm if the rate of 
rise of smoke obscuration exceeds a given value (commonly 0.33 pet m ^ min ^ 
or 0.1 pet ft"''" min"-*-). Auxiliary or backup heat detection is often provided 
with these detectors. 

Photoelectric detectors like ionization detectors may respond to particu- 
late matter from sources other than fire. A dusty mine environment might 
cause some false alarms and would also require periodic cleaning of the sensor 
chamber internal surfaces. In a wet mine, aerosol forms of fog and mists 
might cause some false alarms. These detectors are also somewhat dependent on 
airflow velocity (good chamber design) and location because of the tendency 
for smoke to stratify in air streams. They are also less sensitive to black 
smoke compared with white or grayish smoke. Suppliers of photoelectric smoke 
detectors tend to be the same as those who supply the ionization types; one 
not mentioned previously is the Electro Signal Laboratory. 

Solid-State Detectors 

Solid-state devices for the detection, and sometimes sampling of the 
gaseous components of products of combustion are a third general type of 
detector that can function as a fire detector. 

A recent development in POC gas detection is the semiconductor cell 
detector known as the Taguchi Gas Sensor (TGS) . This device uses a selection 
of bulk n-type metal oxides such as SnOg and FegOs impregnated in a solid- 
state matrix material supported by conductive filaments. Variation of the 
formulation allows the cell to be made more sensitive to a particular gas or 
group of gases and insensitive to others. 

A heater electrode impregnated in the cell provides sufficient increase 
of cell temperature above ambient to allow the cell to operate by diffusion, 
eliminating the need for a sampling pump arrangement. A collector electrode 
provides for measurement of cell conductivity, which is a function of gas 
concentration. The adsorption of a gas molecule on the surface of a semi- 
conductor generally results in the transfer of electrons due to the differing 
energy levels of the gas molecule and the semiconductor surface. Oxygen, 
which can accept electrons, is adsorbed on the surface of n-type semiconductors. 
The transfer of electrons from the donor level of the semiconductor to the 
layer of adsorbed gas results in decreased conductivity of the semiconductor 
material . 



11 



when a TGS that has adsorbed oxygen in this manner comes into contact 
with reducible or combustible gases such as CO, hydrocarbons, etc., the 
molecules of these gases are adsorbed and the transfer of electrons is in the 
opposite direction to the oxygen reaction, releasing the electrons to the 
semiconductor space charge layer and causing a large increase in the conduc- 
tivity of the sensor. The sensor output is sufficiently large to allow gas 
detectors to be designed using a minimum number of components, allowing the 
production of low-cost detectors. 

Significant advantages of these solid-state electrolytic cells are the 
very long life times expected, usually several years, the low cost and sim- 
plicity inherent in its design, and the relatively simplified electronics 
needed to utilize the sensor output. Other features include not being perma- 
nently poisoned by the toxic gases, resistant to vibration and shock, and 
having no loss of sensitivity even at gas concentrations so high that air 
(oxygen) is displaced. 

A severe disadvantage is its ease of alarming in areas containing engine 
combustion products, and its sensitivity to a wide variety of easily oxidized 
gases not necessarily associated with products of combustion. 

For fire detection use the TGS responds readily to CO which is one of the 
principal gases given off in the early stages of fire. However, because of 
its sensitivity to gases other than POC and exhaust gases, its practical use 
may be limited to a hybrid system involving ionization or photoelectric 
detectors. TGS detectors can be calibrated within the range 200 to 1,000 ppra 
CO and to a limited extent can be made selective to CO. 

Electrochemical Detectors 

Electrochemical (fuel cell) techniques for CO sensing are available com- 
mercially from, among others. Energetics Science, Inc., in their CO Ecolyzer, 
Mine Safety Appliance Co., and from InterScan Corp. These devices depend on 
the electrochemical reaction of the detected gas, oxidized or reduced (depend- 
ing on the sensor) at an appropriate electrode potential, producing an elec- 
tric current which, under membrane diffusion controlled conditions, is 
directly proportional to the gas concentration. These devices are subject to 
interference from gases other than that desired for detection, but are capable 
of precision calibration making them useful as an analytical device. 

Recently, compact, portable, and fairly rugged models of electrochemical 
detector systems have come on the market. Some have Mining Health and Safety 
Administration (MSHA) approval for hazardous area use. There has not yet been 
time to thoroughly evaluate and test these models in in situ mine situations 
over extended periods of time and use. 

Optical Gas Detectors 

Optical gas sensors are generally confined to laboratory use, although 
recent prototype models have been developed for personnel hazard monitoring. 
These use the strong adsorption band of CO at 4.65 um (infrared). Radiation 



12 



from an infrared source is passed through a cell through which the sample gas 
flows and is adsorbed by the CO molecule. A filtered infrared detector 
responds to this change in radiation and its output is compared with a refer- 
ence cell, conditioned by suitable electronics and read out on an appropriately 
marked meter. 

A technique useful with different POC detectors is the use of tube bundles 
(7). With these, air samples can be drawn from different sections of the mine 
and sequentially monitored at a central location. This procedure provides pro- 
tection and S'tability for the detector systems and can add to the flexibility 
of the mine fire protection system. Slow response times and wall -diffusion 
losses for submicrometer particles are undesirable features of these systems. 

ENVIRONMENTAL CONSIDERATIONS 

Metal and nonmetal mine environmental extremes can include temperatures 
below 0° to 100° F (-17.8° to 37.8° C) or higher, dry air to 100 pet relative 
humidity, high air velocities --sometimes up to 40 miles/hr (24 km/hr) in ven- 
tilation air intakes, and pressure drops of up to 12 inches (30.5 cm) of water 
across pressure doors. Added to these can be diesel exhaust fumes, shot fir- 
ing gases, and normal mine gases. There are two environmental problems to be 
faced in designing a reliable fire detection system: Will it continue to 
operate, and if it does, can it discriminate against the normal mine ambient 
parameters . 

The latter problem can possibly be dealt with by the use of combinations 
of detector types in the system to provide detection of cellulosic and hydro- 
carbon fires, spontaneous combustion, and electrical fires. Ambient param- 
eters will differ in various parts of the mine. Occasionally air ventilation 
networks can improve a POC detector's sensitivity and effectiveness. Sampling 
tube networks or remote detectors with telemetry can monitor several locations 
individually. A detector type useful in one location may be quite unsuitable 
for use in another location; therefore, if conditions change with time, the 
sensor system should be reevaluated. 

There is no universally applicable fire and smoke detector that responds 
equally well and consistently to all types of fires. The various detector 
types have been discussed along with some to their advantages and disadvan- 
tages. Long-term evaluation of the various detectors' sensitivity and 
reliability under the extremes of ambient conditions just described are under- 
way and will lead to viable, cost effective detection and suppression systems 
for underground mine fires. Until such systems have been developed, and 
environmentally and operationally reliable detectors of various types made 
available through long-term testing, it is best that unreliable fire 
detectors not be installed in mines. 

Ability to discriminate against normal mine ambient parameters is an 
important consideration in the design of reliable fire detection systems. 
Mine Safety Appliances Research Corp. (MSAR) , under a Bureau of Mines contract 
(11) , provided CO, COg, NOx , and NO concentration data for shot firing and 
diesel operations in selected mines and sampling sites within the mine. 



13 



Table 2 taken from their report, summarizes much of their data in the form of 
ratios of contaminants measured in connection with various mine operations. 
The report suggests that a dual sensing system involving contaminant ratios 
may provide an approach to minimizing or eliminating false alarms. The CO/COg 
ratio of diesel exhaust is quite comparable to that of the combustion products 
of a metal mine fire. Similarly, the CO/NOx ratio for shot firing and a leak- 
ing bulkhead, where a hot spot was thought to exist, are also quite similar. 
The COs/NOx ratio is significantly different for shot firing and diesel opera- 
tions compared to the ratio for a fire and/or the leaky bulkhead with a hot 
spot. This much higher last ratio may provide a reliable means of fire 
detection. The report also suggests that studies of the time -distance 
behavior of the NO/NOx ratio be made if the COg/NOx ratio were to be employed 
as a means of incipient fire detection. 

TABLE 2 . - Average contaminate ratios for various 

underground activities 



Activity 

Shot firing 

Diesel 

Fire 

Leaking bulkhead, 4,200 feet, 
Leaking bulkhead, 3,600 feet, 



CO/CO; 



CO/NOx 



COo/NOv 



0.38(0.44) 

.04 
.025 
.03 
.001 



6.35(4.58) 
6.9 
100 

12 



11.6(12.4) 

97 

4,000 

8,000 



Source: Mine Safety Appliances Research Corp. 

In an early Bureau contract study by the Gillette Research Institute (14) . 
two different types of battery powered ionization detectors were taken under- 
ground for a short-term performance evaluation. One, taken into the Bunker 
Hill mine, Kellogg, Idaho, performed poorly after a few hours of operation in 
areas contaminated with high concentrations of aerosols and in regions of high 
ventilation airflows. It did operate satisfactorily in regions supplied by 
low -velocity intake air. 

The other detector, a dual -gate type, performed reliably at Central Rock 
Co., Lexington, Ky. The latter device was not necessarily superior to the 
former because the Bunker Hill mine presented far greater extremes of environ- 
ment. What the experiment tended to demonstrate was that state-of-the-art 
ionization sensors, developed for normal industrial or residential use, 
military or commercial aviation fire protection have questionable reliability 
or, in this case, are inappropriate for use in a mine environment. 



Detector manufacturing companies say that improper maintenance is an 
important contributing factor to the high mechanical and electrical failure 
rate of detectors. On the other hand, recommended cleaning procedures such as 
dust or adsorbed combustion products removal from electrode plates of ioniza- 
tion detectors by way of disassembling the detector head or installing a new 
detector and cleaning the used one aboveground will cause a heavy burden on 
effective maintenance in environmentally extreme regions of the mine. 



14 



These problems, along with those mentioned previously in the discussion 
of POC-type ionization detectors, might lead one to see limited applicability 
for the ionization detector in the mine environment. Yet, by contrast, the 
Becon Mark II ionization detector, also described previously, shows promise 
of long-term reliability both environmentally and operationally. It is cur- 
rently under long-term testing and evaluation. Disadvantages of the Becon 
Mark II are the need for Nuclear Regulatory Commission licensing, because of 
its high beta ionization level, and the need to import it. Domestic models 
with comparable features are under development and show promise of being 
equally effective. 

Another important consideration involving mine environment is the spacing 
and positioning of fire and smoke detectors in the mine. Little is known 
about optimum detector positioning and spacing in mines, NFPA installation 
standards do not offer any criteria directly useful in a mine since their 
codes deal with residential and commercial situations. Mines vary so widely 
in distribution of combustibles, ventilation patterns, and roof support sys- 
tems that an installation type code would not be feasible. Complicated ven- 
tilation patterns, random ceiling obstructions, and a variety of tunnel shapes 
would not allow general applicability of such a code. 

Areas in the mine that represent high risks to personnel and property 
would require increased numbers of detectors per given area. High risk areas 
include fuel storage depots, regions at or near shafts, and abandoned or 
worked -out areas containing combustibles that might ignite spontaneously. 

Direct contact thermal detectors are highly reliable but in some cases 
may not respond rapidly enough. In a fuel storage area, for example, a fire 
could quickly develop to an advanced stage before actuating the extinguishing 
system if only direct contact detectors were used (fig. 2). In this situation 
a flaming fire would likely propagate rapidly, thus calling for a detection- 
actuating system using fast responding optical flame sensors. 

Protection for an underground storage area for other than class B mate- 
rials storage would require a different combination of sensors. In such an 
area, combustibles such as wood timbers and lagging, ventilation ducts, rags, 
cartons, conveyor belts, tires, wood structures, and oxygen -acetylene tanks 
might be found. The normal mine environment, in or near a storage area, could 
produce stimuli that would falsely trigger the different types of fire and 
smoke detectors. Smoking, diesel -powered equipment exhaust, dust, certain 
types of lights, blasting agent residues, high air velocities, and welding and 
cutting gaseous products are examples of contaminants in the normal mine 
environment. An alarm system utilizing smoke detectors to provide early warn- 
ing and a backup system utilizing a thermal wire sensor might be useful in 
this case. 

As previously noted, thermal wire sensors are very reliable and rugged 
but might allow a fire to develop to a fairly advanced stage before signaling. 
With the combustibles mentioned above, a slow, smouldering fire could develop 
and exist for sometime without producing high heat. A smoke detector would 
give an early alarm and would signal personnel in the area or on the surface 



15 




FIGURE 2. - Conceptual fuel storage area prototype fire detection-suppression systems. 

to investigate and take appropriate action. The smoke detector should not 
actuate the extinguishing system automatically because this type of detector 
sometimes alarms in the presence of welding activities, diesel exhaust, or 
excessive dust. The extinguishing system could be heat activated, either 
intrinsically or by the thermal wire sensor; the latter giving an alarm to 
personnel in the area or on the surface. The two types of sensors, connected 
to different audible and visual devices, alarming together would indicate a 
true fire situation. 

A third situation might call for protecting an underground maintenance 
shop area (fig. 3) . Similar combustibles would be found in a shop area as are 
found in a storage area with the likely addition of motor oil, hydraulic fluid, 
and lubricants and their spillage residues. Volatiles such as paints and rub- 
ber or plastic materials are likely to be found there. In addition to these 
possible sources of false fire alarming, cleaning fluids and higher concentra- 
tions of diesel exhaust are likely to be present. 



A dual sensing and extinguishing system of a slightly different type 
might be useful in this area. Thermal wire sensors would provide automatic 
actuation of fire extinguishing equipment and zone isolation as before, and a 
CO detector would provide early warning of fire. The CO detector, possibly a 
TGS type, adjusted to alarm at 50 to 100 ppm would provide a dual service. 
TGS-type sensors are also sensitive to hydrocarbons and will alarm in the 
presence of excessive levels of cleaning solvent vapors as well as CO from 
diesel exhaust, thus providing some protective warning from an air pollution 
s tandpoint . 



16 




FIGURE 3. - Conceptual underground maintenance shop prototype fire detection-suppression 
system. 

Because of the fuel -to -air ratio influence, a knowledge of the relation- 
ship between fire and mine ventilation is essential for the proper selection 
and location of sensors and detectors and also for the correct interpretation 
of information provided by them. Mine fires can be categorized into two 
distinct types --oxygen rich (overventilated) , or fuel rich (underventilated) . 
The fuel -to -air ratio determines which type occurs; the oxygen -rich type can 
evolve into the fuel -rich type by spontaneous growth. Timber fires (fuel -rich) 
represent an extreme toxicity hazard because of low Og concentrations and high 
CO and COg concentrations. Fuel-rich conditions also produce more smoke, 
increasing toxic species exposure, and obscured vision. 



The greatest hazards of mine fires are caused by toxic and sometimes 
explosive products of combustion being carried through the mine by the ventila- 
tion system and unexpected airflow reversals carrying toxic fumes to intake 
air ventilation areas, such as fire escape routes, hoist areas, and other 
areas usually thought of as safe in the event of a fire. The analytical pre- 
diction of airflow reversal in a mine, even under ideal conditions, is a very 
difficult task and beyond the scope of this report. However, Greuer (6) has 
compiled a large number of references and provided a detailed accounting of 
the influence of mine fires on the ventilation of underground mines for those 
who wish to pursue the problem in detail. 



17 



Greuer (6) makes a few quantitative predictions of ventilation disturb- 
ances that may be useful in interpreting the indications seen when monitoring 
a fire detection system that includes the following: 

1. Throttling and natural draft changes can cause changes in the quan- 
tity of ventilating air currents and sometimes a reversal of their direction. 
Similar changes can occur in neighboring connected airways as well as that of 
the fire scene. 

2. Air (oxygen) quantity reductions (Og pet) in nongassy mines can be 
neglected. In gassy mines air reductions can lead to explosive mixtures that 
may be carried back through the fire zone. 

3. Ventilation disturbances can also cause smoke layering which may 
affect the ability of a detector to operate effectively. 

BUREAU OF MINES EXPERIENCE 

Shaft Installations 

To work towards an answer to the major problem of fire hazard and con- 
taminated air resulting from fire, the Federal Bureau of Mines contracted with 
the FMC Corp., San Jose, Calif., to evaluate mine shaft fires and their hazards . 
(5) . The contract also called for the development and demonstration of a low 
cost, reliable mine shaft fire protection system that could be adapted to a 
majority of metal and nonmetal mine shafts, raises, and winzes, especially in 
deep mines. Results of this contract have been published elsewhere (5) so 
need not be detailed here. However, as an example of a proven system, sensor 
and detector components that performed successfully in underground tests with 
controlled fires will be highlighted here. 

As part of the background preparation for designing the systems produced, 
it was found that approximately 67 pet of shafts in metal and nonmetal mines 
are of wooden construction, and contaminated air was responsible for about 
77 pet of fatalities from fires. Causes of fires in mines were predominantly 
electrical (35 pet) and welding (18 pet). Most fire -connected fatalities 
occurred in or near shaft stations, the gathering point to normally leave the 
mine. 

Four major needs were brought out for providing effective mine fire and 
smoke protection: 

1. Reliable sensors are necessary for early fire and smoke warning. 

2. Isolation barriers (vent doors) would stop the flow of contaminated 
air. 

3. Available water supplies could assist with fire extinguishing. 

4. Recording sensor systems and control of ventilation doors and extin- 
guishing systems from the surface would be preferable to sending firefighting 
teams underground. 



18 



The possibility of a fire starting somewhere in the mine is always pres- 
ent because combustibles and ignition sources are always close together. In 
addition to constant fire -prevention efforts, provisions should be made to 
detect and extinguish fires as soon as possible. Sensors and detectors are 
needed because fires can occur spontaneously in abandoned workings or acci- 
dently in unattended areas, and depending on detection by persons working in 
or passing through an area is not realistic. 

Besides simply detecting a fire, provision should be made to alert an 
attended area that fire exists and its location, to alert people underground 
that a fire has been observed or reported, to take some action to isolate the 
fire area, and to extinguish the fire if possible. Meeting these requirements 
was possible with existing equipment integrated into a functional protective 
system consisting of sensors, extinguishing agents, isolation barriers, and 
appropriate controls . 

The harsh environmental conditions that exist underground have already 
been mentioned. Because labor and maintenance efforts are at a premium under- 
ground, all components used in this detection-alarm system must be extremely 
rugged to limit repair needs or they may be out of service at just the wrong 
times, and they must be reliable to instill confidence in their ability to 
perform and alarm only in a fire situation. 

The first prototype mine shaft fire and smoke protection system was 
tested at an inactive silver mine in the Couer D'Alene district of Idaho. The 
system was installed 3,000 feet below the shaft collar to protect 50 feet of 
shaft, the shaft station, and 100 feet of drift in two directions. 

Environmental conditions in the mine were 100° F and essentially 100 pet 
humidity. Because the mine was inactive, there was little or no dust present 
during the 2 -week test period. Ventilation was not typical of that found in a 
working mine either because the mine was used exclusively for ventilation and 
as an alternate escape route for an adjoining working mine. Airflow was tem- 
porarily modified for the test. 

The sensors installed included thermal wire, CO gas, and ionization types. 
The thermal wire provided backup reliability for the other two early warning 
types . Carbon monoxide gas and ionization sensors were placed in sections of 
the shaft and shaft station areas. A pair of each was placed near the shaft 
crown plate to sense air entering the area, which was downcast intake air. 
Others were located near the ventilation barriers towards the mine workings 
(away from the shaft station area). 

Sensors were selected to respond to health hazard levels of fire or con- 
tamination by fire (gas-smoke), while being available off-the-shelf, reason- 
ably priced, and compatible with metal -nonmetal mine environments as previously 
described. Sensors are pictured in figure 4, and consist of smoke (ionization) 
detectors, Becon Mark II, manufactured by Anglo American Electronics, Republic 
of South Africa; two carbon monoxide (gas) sensors, model CO-181 manufactured 
by Dynamiation Enterprises, Inc., Ann Arbor, Mich.; and model MSD-1 manufac- 
tured by Enmet Co., Ann Arbor, Mich.; and thermal wire, type WPP, 155° F manu- 
factured by the Protectowire Co. 



19 




FIGURE 4. - Smoke (A), gas (b), and heat (c) sensors. 

The ionization detectors are referred to as smoke detectors in the system 
even though they primarily sense invisible submicrometer particles (as distin- 
guished from the second detector type, labeled CO, which responds to CO and 
other components of smoke) . 

During extensive factory tests, the ionization detectors performed well. 
They consistently detected wood fire smoke within habitable levels. Test data 
indicate the detectors alarmed at smoke density levels of 2 to 6 pet obscura- 
tion per foot. This compares favorably with industry standard set points for 
commercial and residential smoke detectors. A poor responoe to plastic pro- 
duced smokes appears to be characteristic of ionization -type detectors, and 
the Becon unit is no exception. However, the Becon unit is designed for mine 
environments and does not readily exhibit problems from corrosive environments, 
dust, humidity, and very high air velocities. 



The CO detectors in the system use the Taguchi gas sensor (TGS) . They 
were used at factory settings of 200 ppm CO on the Enmet unit and 75 ppm CO 
on the Dynamation units. These levels were felt to be realistic for the con- 
ditions expected in the test shaft. There was no blasting or diesel equipment 
operating to increase the CO level at the demonstration site. On the other 
hand, a CO concentration of 150 ppm may be encountered with some regularity in 



20 



active mines where blasting and diesel equipment are present. A level of 
150 ppm is tolerable without hazard for up to 3 hours, thus setting alarm 
points of CO monitors above 150 ppm would represent a realistic alarm level 
within the limits of safe habitation for short periods, but an extended 
200 ppm concentration could be a valid indication of a fire, especially if 
combined with other indications. 

A surface control unit was installed in the hoist room and connected to 
an underground control unit in the shaft station at the 3,000-foot level. The 
latter was the junction point of the sensors and related components and pro- 
vided the interface between them and the surface control unit. The two con- 
trol units were connected by a single pair of wires with a second pair for 
redundancy. Multiplexing the signals allowed the surface control unit to 
handle 16 inputs from the underground control and process 8 outputs for trans- 
mittal back to the underground unit. 

A fire fueled by wood cribbing was built in a steel pan 3 feet square, 
about 18 inches deep, with a lid that was raised and lowered to adjust smoke 
density and products of combustion. 

Smoke and CO sensors responded to contaminants in the fire area, all 
system commands were remotely operated from the surface control unit and per- 
formed as designed. The fire was extinguished by the installed sprinkler 
system during a 6 -minute discharge. The thermal wire was located too high 
above the controlled pan fire to reach its alarm temperature and did not 
respond. 

The first prototype system was installed for only 2 weeks in the Idaho 
mine which did not test the long-term reliability of the system and components, 
A second system was installed in Union Carbide Corp.'s Pine Creek mine in 
Bishop, Calif., where it could be given a larger test. 

Several modifications were made to adapt the system to the particular 
mine location and it was installed and continuous monitoring began on 
December 19, 1975. A second prototype system based on recommendations derived 
from developing, installing, and monitoring the first prototype, was built and 
installed at a second level in the Pine Creek mine as an add-on-unit to the 
first prototype using the same interconnecting wires. The latter system 
included a new microprocessor controlled surface unit, replacing the original 
prototype surface unit. System testing was concluded in July after controlled 
fire test demonstrations were performed in each of the two underground areas. 
Final results include 3 months operation of the first prototype system, provid- 
ing 87 days of 24 -hour -per -day monitoring. The second prototype system was 
installed 3-1/2 months, providing a combined total of 105 additional days of 
monitoring for both levels. 

The surface control unit was installed on the surface at a guardhouse 
where it was monitored 24 hours per day. The fire and smoke protection sys- 
tems were placed at two locations deep within the mine --one in a large com- 
pressor station 14,000 feet in the mine at the 9, 400 -foot level, and the 
second 14,000 feet in the mine at a shaft and shaft station area at the 
11,271-foot level. Because the mine is inverted, deeper operations are at 
higher elevations. 



21 



Three types of sensors were used at each of the two test sites as in the 
initial test setup. Thermal wire was strung throughout the drift and machin- 
ery space of the compressor station and around and down the shaft at the 
11,271-foot level. 

Ionization and CO sensors were used in pairs except none of the CO sensors 
were used in the compressor room. An ionization sensor was located above the 
compressor machinery, one located on each side away from the compressor sta- 
tion, in intake air, beyond remotely controlled vent doors, and one in the 
upcast exhaust airflow from the shaft at the 11, 271 -foot level. An additional 
ionization sensor was placed in the diesel maintenance area, three fans were 
monitored, one was controlled, and air lock doors were monitored. 

During the monitoring period, the smoke (ionization) detector in the 
diesel maintenance bay regularly alarmed during vehicle activity, requiring 
an adjustment of its sensitivity. Also the smoke detector in the compressor 
station was readjusted because of alarms. The sensors responded to various 
known mine activities --diesel exhaust, blasting, and gas and electric welding 
and cutting vapors . 

After the monitoring period each location was subjected to a test fire 
using a pan and cribs identical to those used in the Silver Summit test. At 
the 11, 271 -foot level it was allowed to burn for nearly 12 minutes before the 
surface control unit was used to turn on the installed sprinkler system. Dur- 
ing the fire the sensors detected over 100 ppm gas and smoke buildup, and 
alerted surface personnel that they were in alarm. All alarming and control 
functions performed satisfactorily through the surface control unit operated 
by surface personnel. 

At the 9,400-foot level, a similar fire was allowed to burn for 13 min- 
utes before the surface control unit turned on the sprinkler system. Again, 
alarm and control functions operated satisfactorily through the surface con- 
trol unit, responding to the buildup of smoke. 

Underground Fueling Area Installation 

A prototype underground fueling area system was designed, tested, and is 
now undergoing long-term testing at an underground fueling and storage area of 
the Union Carbide Corp.'s Pine Creek Mine. This was developed and installed 
under a contract to the Ansul Co., Marinette, Wis. (2^), to develop safe prac- 
tice guidelines that minimize the chances of fire in underground fueling areas 
and to develop a low-cost, reliable, automatic fire protection system for 
underground fueling areas . 

The detection and control subsystem contains two ultraviolet -type 
detectors monitored by a control panel that provides a pneumatic output to 
the suppression subsystem when flames are present within the cone of vision 
of the detectors. The detectors are self-contained units that accept 24 vdc 
and provide both instantaneous and time-delayed form C contacts as outputs. 
The control system uses the 5 -second delay contacts as a zone alarm input. 
The detectors respond to ultraviolet and include an optical integrity feature 



22 



that allows remote testing of their optical lenses. The detectors are housed 
in explosion-proof enclosures and placed in the fueling area such that their 
90° fields of vision overlap and each covers the complete area. 

The two detectors are cross -zoned within the control unit to minimize 
false alarms. This means that both detectors must sense the fire before the 
fire signal passes to the actuation device. The fire signal operates a 
mechanical releasing device that releases an internally stored pressurized gas 
to operate the suppression system. The control unit has an internal, adjusta- 
ble time delay to delay actuation of the suppression system at a predetermined 
time after the alarm signal is received. This allows the use of an "abort" 
function through the control unit to prevent system discharge in the event of 
a false, or nuisance alarm, and also allows the use of predischarge alarms. 
The control unit has additional relay contacts to allow for external ancillary 
functions and remote annunciation: however, the prototype system includes only 
local alarms . 

The control unit is powered directly from an emergency power source which 
converts primary ac power to low-voltage dc power for the control unit and 
also maintains a float -charged secondary standby battery source. The standby 
batteries have sufficient energy storage capacity to operate the entire system 
for a period of at least 24 hours. 

The total system was tested on a component basis as well as on a system 
basis before installation in the mine. A field demonstration was performed at 
the mine . 

Underground installation of the detection and control subsystem was com- 
pleted first and detector performance was monitored for several days before 
the demonstration. During 3 days of monitoring, no false alarms or other 
problems occurred. On the fourth day, a malfunction was discovered in the 
unused portion of the time-delay circuit that was caused by the temperature- 
humidity effect of the underground atmosphere. A new circuit board was 
installed in that portion of the subsystem. 

On July 31, 1977, two underground fire tests were performed to determine 
the discharge characteristics of the system and to check the coverage of the 
fire suppressant. These were entirely satisfactory and follow-on, long-term 
validation testing of the prototype hardware has continued at Pine Creek mine. 
During the past nearly 20 months of in -mine testing, there have been no func- 
tional failures of the system, although the detector lenses have required an 
occasional cleaning. 

Additional Underground Tests 

Responses to fires from different fuels, nonfire stimuli, effects of air 
velocity variations, dust buildup, and effects of mine environments are of 
interest over long-term testing. 

Therefore, the Bureau's Twin Cities Mining Research Center entered into a 
purchase contract with the FMC Corp. to obtain a sensor package suitable for 



23 



Smoke 
sensor 



Chart 
recorder 



120-vac power 

CT 



o 



CO 
sensor 



Optical 
sensors 

HZ3 



Phone 
line 




120-vac power 



long-term, in -mine testing. 
A cooperative agreement for 
locating and testing the 
package was also negotiated 
with the Hecla Mining Co., 
Lakeshore mine near Casa 
Grande, Ariz. 

A three -sensor package 
containing a Becon Mark II 
smoke sensor, an Enmet ISA-33 
CO gas sensor, and two Pyro- 
tector 30-2013-9 flame 
detectors were assembled 
(fig. 5). An additional CO 
gas sensor (Dynamation CO 
181) was supplied for addi- 
tional testing at a differ- 
ent location in the mine. 

The three -sensor pack- 
age was installed in the 
underground diesel fuel 
storage area located at the 
end of the 500 east exhaust 
crosscut. The single CO 
gas sensor was installed in 
the south decline near the 
mine's existing Ecolyzer gas 
sensor. Each sensor was connected by telephone wire pairs to a nine -channel 
recorder on the surface. These packages were installed in late March 1976, 
and have been undergoing long-term testing. 

Periodic visits to the mine to observe the sensors indicated some prob- 
lems. The interface electronics and the CO sensors are packaged in NEMA IV 
electrical enclosures and well -protected from the mine environment. The TGS 
sensors that are part of the CO monitors are not stable in that their base level 
output drifts over time and with temperature variations. They are also quite 
sensitive to diesel exhaust emissions and blasting residue gases in the mine. 

The flame detectors remained operational but they became dust covered 
with time since they are in an exhaust crosscut where the air velocity is 
quite high and dusty. 

The smoke sensor remained operational but after several months was not 
alarming as it should. Blowing accumulated dust away from its baffle 
entrances caused it to go into alarm, and it could be made to alarm by check- 
ing its trigger level adjustment. Dust accumulated in the baffle area in this 
location to the extent it needed to be cleaned out every few months. The 
alarm level was adjusted later so it would not respond to the exhaust fumes 
from standing, idling load -haul -dumps in the passageway to the fueling areas. 
Additional long-term testing of sensors will be continued in mines with differ- 
ing environmental conditions. 



FIGURE 5o - Underground metal mine fire sensor package diagram. 



24 



REFERENCES 

1. Astheimer, R. W., and J. Schwartz. Thermal Imaging Using Pyroelectric 

Detectors. Appl. Opt., v. 7, No. 9, September 1968, pp. 1687-1695. 

2. Christensen, B. C, and G. R. Reid. Improved Fire Protection System for 

Underground Fueling Areas. Final Report, Volume I (Research Contract 
No. H0262023). BuMines Open File Rept. 120-78, 1977, 325 pp; available 
for consultation at the Bureau of Mines Libraries in Pittsburgh, Pa., 
Minneapolis, Minn., Denver, Colo., and Spokane, Wash.; at the National 
Library of Natural Resources, U.S. Department of the Interior, Washing- 
ton, D.C.; and from the National Technical Information Service, Spring- 
field, Va., PB 288 298/AS. 

3. Cohen, J., S. Edelman, and C. F. Vezzetti. Pyroelectric Effect in Poly- 

vinylflouride. Nature (London) Phys . Sci., v. 233, No. 36, Sept. 6, 1971, 
p. 12a. 

4. Drinker, P., and T. Hatch. Industrial Dust. McGraw-Hill Book Co., New 

York, 1936, p. 7. 

5. FMC Corporation. Mine Shaft Fire and Smoke Protection System (Final 

Report). Volume I. Design and Demonstration (Research Contract 
No. H0242016). BuMines Open File Rept. 24-77, July 1975, 407 pp.; 
available for reference at Bureau of Mines Libraries in Denver, Colo., 
Twin Cities, Minn., Bruceton and Pittsburgh, Pa., Spokane, Wash.; 
U.S. Department of Energy, Morgantown Energy Research Center, Morgan- 
town, W. Va.; The National Library of Natural Resources, U.S. Depart- 
ment of the Interior, Washington, D.C.; and from National Technical 
Information Service, Springfield, Va., PB 263 577/AS. 

6. Greuer, R. E. Influence of Mine Fires on the Ventilation of Underground 

Mines (Research Contract No. S0122095) . BuMines Open File Rept. 74-73, 
1973, 173 pp.; available for consultation at the Bureau of Mines 
libraries in Pittsburgh, Pa., Minneapolis, Minn., Denver, Colo., and 
Spokane, Wash.; at the National Library of Natural Resources, U.S. 
U.S. Department of the Interior, Washington, D.C.; and from the 
National Technical Information Service, Springfield, Va.; PB 255 834/AS. 

7. Hertzberg, M. , and C. D. Litton. Multipoint Detection of Products of Com- 

bustion With Tube Bundles. Transit Times, Transmissions of Submicrom- 
eter Particulates, and General Applicability. BuMines BlI 8171, 1976, 
40 pp. 

8. Litton, C. D. , and M. Hertzberg. Principles of Ionization Smoke Detec- 

tion. Development of a New Sensor for Combustion -Generated Submicrom- 
eter Particulates. BuMines RI 8242, 1977, 21 pp. 

9. National Bureau of Standards. Fire Detection, the State of the Art. 

NBS Tech. Note 839, June 1974, 56 pp. 

10. National Fire Protection Association. Protection Handbook. 13th ed., 
1968, 1946 pp. 



25 



11 



Rogers, S. J. Analysis of Noncoal Mine Atmospheres (Research Contract 
No. S0231057). BuMines Open File Rept. 78-77, 1976, 121 pp.; available 
for consultation at the Bureau of Mines libraries in Pittsburgh, Pa., 
Denver, Colo., Minneapolis, Minn., and Spokane, Wash.; at the National 
Library of Natural Resources, U.S. Department of the Interior, Washing- 
ton, D.C; and from National Technical Information Service, Springfield. 
Va. , PB 266 764/AS. 



12. Verdin, A. Gas Analysis Instrumentation. 
York, 1973, 414 pp. 



John Wiley and Sons, Inc., New 



13. Wagner, J. P., A. Fookson, A. Harper, M. May, and R. Welker. Fire Alert 

Systems for Metal and Nonmetal Mines (Research Contract No. S0144131) . 
BuMines Open File Rept. 45-76, 1975, 163 pp.; available for consulta- 
tion at the Bureau of Mines libraries in Pittsburgh, Pa., Minneapolis, 
Minn., Denver, Colo., and Spokane, Wash.; at the National Library of 
Natural Resources, U.S. Department of the Interior, Washington, D.C; 
and from the National Technical Information Service, Springfield, Va., 
PB 251 715/AS. 

14. Wagner, J. P., and A. Fookson. Application of Fire/Cas Sensor Detection 

Technology to Metal and Nonmetal Mine Fire Problems. Fire Res . Absts . 
and Reviews, National Academy of Sciences , v. 17, Nos . 1-3, 1975, p. 66. 



i!rU.S. GOVERNMENT PRINTING OFFICE: 1979-603-002/114 



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